drm/xe: Explicitly mark migration queues with flag

Rather than inferring if an exec queue is a migration queue for a flag,
explicitly mark migration queues with a flag.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Link: https://lore.kernel.org/r/20250724191216.4076566-2-matthew.brost@intel.com